1. Field of the Invention
The present invention relates to a predistortion-based amplifier for compensating for distortion generated thereby when it amplifies a modulated wave signal based on envelope information of the modulated wave signal in accordance with a predistortion scheme, and more particularly, to a predistortion-based amplifier for improving an envelope information detecting efficiency and detecting accuracy.
2. Description of the Related Art
The predistortion scheme, which is known as a technique for linearizing amplifiers, detects an envelope using an envelope detector to compensate for distortions generated by an amplifier based on the result of the detection.
The predistortion scheme will now be described.
In a power amplifier used in a transmitter, its input/output characteristics include nonlinearity which causes distortions, resulting in unnecessary power radiation to the outside of a transmission frequency band. This causes interfering power to other frequency bands. For this reason, such a power amplifier is required to minimize the distortions generated thereby.
Also, since the power amplifier is a component which consumes large power, a reduction in power consumption of the power amplifier is regarded as an important issue because in a base station unit of a portable telephone system or the like, for example, a power amplifier consumes a majority of the power consumed by the base station unit.
However, for realizing a linear amplification using a power amplifier, it is necessary to operate the power amplifier in a region in which a direct current-to-alternating current converting efficiency (in this disclosure, hereinafter simply referred to as the xe2x80x9cefficiencyxe2x80x9d) is low. In other words, an improvement on the nonlinearity of the power amplifier and a reduction in power consumption are in a mutually conflicting relationship, so that it is a difficult challenge to simultaneously solve the two problems.
As a method for simultaneously solving the two problems as mentioned, the predistortion scheme has been proposed.
The predistortion scheme is one approach to reduce distortions which applies, for example, distortions opposite to those generated by a power amplifier to a stage previous to an input of the power amplifier (distortion for canceling the distortions generated by the power amplifier) to improve the distortion characteristic of the power amplifier. In addition to this predistortion scheme, a scheme which applies such opposite distortions in a baseband is referred to as a baseband predistortion scheme; a scheme which applies such opposite distortions in an intermediate frequency (IF) band is referred to as an IF predistortion scheme; and a scheme which applies such opposite distortions in a radio frequency (RF) band is referred to as an RF predistortion scheme.
Generally, the nonlinearity found in transmission power amplifiers is divided into an amplitude component and a phase component. Then, the nonlinearity in the amplitude component is generally represented by how the gain changes with respect to input power supplied to a power amplifier, referred to as an AM (Amplitude Modulation)xe2x80x94AM characteristic. The nonlinearity in the phase component in turn is represented by how the phase rotates with respect to the input power supplied to the power amplifier, referred to as an AM-PM (Phase Modulation) characteristic.
The predistortion scheme as mentioned above uses, for example, a voltage controlled variable attenuator to generate distortions having an opposite characteristic to the AM-AM characteristic of a power amplifier at a stage previous to the power amplifier, and uses a voltage controlled variable phaser to generate distortions having an opposite characteristic to the AM-PM characteristic at a stage previous to the power amplifier, thereby reducing distortions in amplitude and phase generated by the power amplifier.
In the RF predistortion scheme and IF predistortion scheme, for example, a variable attenuator and a variable phase must be operated in accordance with the envelope of a high frequency signal which is to be amplified. Generally, with a transmission bandwidth of 20 MHz, by way of example, the envelope has a frequency component of 20 MHz at maximum. Therefore, such a predistortion scheme requires an envelope information detector circuit which has an input bandwidth as wide as the transmission bandwidth.
Exemplary configurations of circuits using the predistortion schemes as mentioned above are disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-69098 and Japanese Patent Application No. 2000-260214.
Next, the envelope information detector used in the aforementioned RF predistortion scheme and IF predistortion scheme will be described.
FIG. 9 illustrates an exemplary circuit configuration of an envelope information detector which may be included in a conventional predistortion-based amplifier. The illustrated envelope information detector comprises a matching capacitor 41, a diode 42, a matching coil 43, an RF choke coil 44, a biasing voltage source 45, a capacitor 46, and a resistor 47. For the diode 42, a Schottky diode, for example, is employed because of its ability to operate at a high speed and detect high frequencies.
Specifically, in the envelope information detector illustrated in FIG. 9, an input terminal D1 which receives a modulated wave signal is connected to one end of the matching capacitor 41, the other end of which is connected to an anode of the diode 42. A cathode of the diode 42 is connected to an output terminal D2. A point between the input terminal D1 and the matching capacitor 41 is grounded through the matching coil 43, and the biasing voltage source 45 is connected between the matching capacitor 41 and the diode 42 through the RF choke coil 44. Also, one end of the capacitor 46 and one end of the resistor 47 are connected in parallel between the diode 42 and the output terminal D2, and the other end of the capacitor 46 and the other end of the resistor 47 are grounded.
Also, in the envelope information detector illustrated in FIG. 9, the matching capacitor 41 and the matching coil 43 constitute a matching circuit, while the capacitor 46 and the resistor 47 constitute a low pass filter (LPF).
An exemplary operation of the envelope information detector illustrated in FIG. 9 will be described below.
In the envelope information detector, a modulated wave signal input to the input terminal D1 is applied to the anode of the diode 42 through the matching circuit. At the anode of the diode 42, the input modulated wave signal is provided as a voltage signal. Also, for example, if the capacitor 46 has a sufficiently large capacitance with respect to the carrier frequency of the modulated wave signal so that it is regarded as being short-circuited at the carrier frequency in the AC domain, the modulated wave signal provided at the anode of the diode 42 is converted to a current signal by the diode 42.
Simultaneously, the modulated wave signal is rectified by the ON-OFF characteristic of the diode 42, whereby the modulated wave signal is separated into a carrier component and an envelope component. Then, the carrier component included in the converted current signal is removed by the LPF, so that the envelope component alone is detected by the resistor 47 as a voltage signal which is output from the output terminal D2.
With the operation as described above, the envelope information detector detects the envelope component of the input modulated wave signal.
As an example of conventional techniques, a modulated output detector circuit for a radio device described in Japanese Patent Application Laid-Open No. 5-251971 uses a quarter wavelength transmission path which functions as a diode or a filter for detecting a modulated output from a high power amplifier, resistor for leading out a detected output, and so on in an ALC (Automatic Level Control) circuit for controlling the transmission power to a predetermined state to input a predetermined detected output to a control signal detected output converter circuit, thereby adjusting the amount of attenuation of the input to the high power amplifier to hold the output from the high power amplifier at a predetermined level.
However, in the envelope information detector for the conventional predistortion-based amplifier as illustrated in FIG. 9, for example, although the envelope detecting efficiency can be improved by increasing the capacitance of the capacitor 46 and the resistance of the resistor 47 connected to the output terminal of the diode 42, the increase in the capacitance of the capacitor 46 and the resistance of the resistor 47 results in a larger time constant to lose high frequency components within the envelope component, so that a detected envelope is corrupted, in other words, the envelope detecting accuracy is deteriorated. As appreciated, in the conventional envelope information detector, the envelope detecting efficiency and detecting accuracy are in a mutually conflicting relationship, thus encountering difficulties in simultaneously improving both. Thus, the conventional predistortion-based amplifier is problematic in that a distortion compensating accuracy and so on are worsened.
The present invention has been made to solve the problems of the prior art as described above, and its object is to provide a predistortion-based amplifier which is capable of improving the envelope information detecting efficiency and detecting accuracy for compensating for distortions generated by the amplifier, when it amplifies a modulated wave signal, based on envelope information of the modulated wave signal in accordance with a predistortion scheme.
To achieve the above object, the predistortion-based amplifier according to the present invention forms an envelope information detector using a stub circuit for removing a carrier component of a modulated wave signal. Distributing means distributes the modulated wave signal to be amplified, distortion generating means generates distortions in at least one of amplitude and phase of one distributed signal, the envelope information detector detects information related to the envelope of the other distributed signal when the amplifier amplifies the one distributed signal in which distortions have been generated by the distortion generating means, and distortion control means controls the distortions generated by the distortion generating means based on the result of a detection by the envelope information detector such that distortions generated by the amplifier is compensated for.
Since the carrier component of the modulated wave signal is removed by the stub circuit, the envelope information detector can improve both the envelope information detecting efficiency and detecting accuracy, thereby making it possible to improve a distortion compensating accuracy and so on in accordance with the predistortion scheme.
Here, as modulated signals amplified by the amplifier, a variety of signals may be used.
The distortion generating means may generate, for example, both amplitude distortions and phase distortions, or may generate, for example, only amplitude distortions, or may generate, for example, only phase distortions. Means for generating amplitude distortions may be implemented, for example, by amplitude changing means such as a variable attenuator which changes the amplitude of a signal. Means for generating phase distortions may be implemented, for example, by phase changing means such as a variable phaser which changes the phase of a signal.
As the information related to the envelope (envelope information) detected by the envelope information detector, a variety of information may be used. For example, continuous or discrete detection of information on the level of the envelope, and so on may be used. Specifically, in the predistortion-based amplifier, the envelope information detector may detect information which enables the predistortion-based distortion compensation to be effectively carried out in practice.
The control for compensating distortions generated by the amplifier (for reducing the distortions) is preferably conducted, for example, to suppress the distortions generated by the amplifier to a minimum (ideally, zero). However, another manner of control may be used as long as it is effective in a practical use.
The distortions generated by the distortion generating means may be controlled, for example, by controlling the amount of amplitude distortions or phase distortions generated by the distortion generating means.
As the stub circuit, a variety of circuits may be used. Specifically, a circuit comprised only of a stub, for example, may be used, or a circuit comprised of a stub and another circuit may be used. Also, as the shape of the stub, a variety of shapes may be used.
The stub circuit may remove the carrier component of a modulated wave signal to such a degree that envelope information can be effectively detected in practice, so that the carrier component need not be suppressed to zero.
In the predistortion-based amplifier according to the present invention, the envelope information detector includes a diode circuit for rectifying the other distributed signal, and a stub circuit for removing a carrier component of the signal output from the diode circuit (the carrier component of the modulated wave signal), as a more specific exemplary configuration.
With this configuration, the envelope information detecting efficiency can be improved in accordance with the characteristic of the stub circuit, and the envelope information detecting accuracy can be improved in accordance with the characteristic of the diode circuit, thereby making it possible to improve both the envelope information detecting efficiency and detecting accuracy.
Here, as the diode circuit, a variety of circuits may be used. Specifically, a circuit comprised only of a diode, for example, may be used, or a circuit comprised of a diode and another circuit may be used.
In the predistortion-based amplifier according to the present invention, the envelope information detector includes a matching circuit disposed at a stage previous to the diode circuit, and a signal converter circuit for converting an envelope signal output from the stub circuit from a current signal to a voltage signal, as a more specific exemplary configuration.
Also, in the predistortion-based amplifier according to the present invention, as a preferred exemplary configuration, the stub circuit of the envelope information detector is implemented as a notch filter using an L-shaped microstrip line, one side of which is used as a stub portion.
Further, in the predistortion-based amplifier according to the present invention, the stub circuit of the envelope information detector has a plurality of stub portions, in which case an impedance can be minimized, for example, in a frequency range having certain width, as shown in an embodiment, later described.
Therefore, even when the frequency band of a modulated wave signal to be amplified has certain width, the envelope information detecting efficiency and detecting accuracy can be improved in such a frequency region having certain width, thereby making it possible to improve the distortion compensating accuracy and so on in accordance with the predistortion scheme.
Here, the number of stub portions included in the stub circuit may be a variety of numbers depending, for example, on a situation in which a system is used, and so on.
In the predistortion-based amplifier according to the present invention, as a preferred exemplary configuration, the stub circuit of the envelope information detector is implemented as a band elimination filter (BEF) using an inverted C-shaped microstrip line, in which case two opposing sides of the C-shaped microstrip line are used as stub portions, respectively.
Also, in the predistortion-based amplifier according to the present invention, as a preferred exemplary configuration, the stub portions included in the stub circuit of the envelope information detector has a length chosen to be equal to one quarter (or approximately one quarter) of the wavelength of the carrier component of the modulated wave signal.
The predistortion-based amplifier according to the present invention is preferably applied, for example, to a transmitter.
Specifically, a transmitter according to the present invention comprises a predistortion-based amplifier having a variety of configurations as shown above, and amplifies a modulated wave signal to be transmitted using the ppredistortion-based amplifier before transmission.
The signal maybe transmitted, for example, through a wired communication, or for example, through a radio communication.
The predistortion-based amplifier including a stub circuit having a plurality of stub portions is preferably applied to a transmitter for use in a mobile radio communication system which employs, for example, a CDMA (code division multiple access) scheme such as a W-CDMA (Wideband-Code Division Multiple Access) scheme for transmitting a wideband signal.
Specifically, a CDMA transmitter according to the present invention includes a predistortion-based amplifier including a stub circuit having a plurality of stub portions, and amplifies a modulated wave signal to be transmitted using the predistortion-based amplifier before transmission.
Here, as the mobile radio communication system, a variety of systems, for example, a portable telephone system and so on may be used.
As described above, the envelope information detector included in the predistortion-based amplifier according to the present invention detects envelope information by a method of detecting information related to the envelope of a modulated wave signal using the stub circuit for removing the carrier component of the modulated wave signal, thereby realizing the detection of the envelope information with a high detecting efficiency and a high detecting accuracy.
As an example, an envelope detector according to the present invention includes a diode circuit for rectifying a modulated wave signal, and a stub circuit for removing a carrier component of a signal output from the diode circuit to detect information related to the envelope of the modulated wave signal, thereby realizing the advantages as described above.